The Debug -> Requests tab is displayed only when request debugging is enabled. This can be achieved either by configuring certain settings in Site Manager -> Settings -> System -> Debug, or by adding specific keys to the appSettings section of your project's web.config file. Both options are described in this context help topic.
On this tab, you can see the time each of the recent page requests took to process. You not only see the overall time, but also separate times of particular parts of each requests, along with other detailed information about the request.
This is particularly useful if your response time is too large and you need to figure out why. You can basically see whether the issue is outside the application or inside it by comparing the real response time and response time spent in the application, what data came with the request and from which context.
Enabling the Show complete context check-box displays complete context (not only the topmost item) in the Context column. The log can be cleared using the Clear request log button.
For more information about debugging in Kentico CMS, please refer to Developer's Guide -> Development -> Debugging and system information.